Abbe type refractometer



New.,l 13, 1951 J. W. FORREST ET AL ABBE TYPE REFRACTOMETER Filed July 20, 1949 v 2 SHEETS-SHEET 1 JOHN W. FORREST R. F. E. ST EGEMAN Nov. 13 1951- ABBE TYPE REFRACTOMETER Filed July 20, 1949 2 SHEETS-SHEET 2 f I lo Lui; g-HmqV-'lh uml 2, 27 Jil?. "Il l@ Hfl I IIE; nl/54 ""uu'ylam '1 ,um 82 l 68 9o Wl @2 :,III f 89 'fl-Tf1 9| o im lll/,/ k all '(92 INVENToR.

JOHN W. FORREST R.F.E. STEGEMAN Patented Nov. 13, 1951 ABB TYPE REFRACTOMETER John W. Forrest, Brighton, andA Raymond F. E. Stegeman, Greece, N. Y., assignors to Bausch & Lomb Optical Company, Rochester, N. Y., a

corporation of New York Application July 20, 1949, Serial No. 105,682

3 Claims. 1

This invention relates to refractometers of the Abbe type and more particularly it relates to improvements in the operating mechanism, achromatizing means and the general housing structure of the instrument.

Due to the increasing use of refractometers as production testing instruments in diverse and exacting elds of use, need has arisen for a refractometer which is rugged enough in construction to withstand 'careless handling in production operations While still being capable of continuous precision performance over long periods of use.

Because refractometers are used to test a Wide variety of substances, such as food products, oil, etc., these instruments become damaged as a result of the adherence of such substances to the working parts thereof. It is, therefore, impor- `\tant that these instruments are so designed that the `working lparts are protected from the injurious effects of such deposits without interfering with accurate and expeditious use of the instrument.

y One of the essential elements used in obtaining precision performance from the Abbe type refractometer is the compensating prism by means of which the dividing line of total reflection in the eld of view is achromatized when using White light. These prisms are of the Amici type and in manufacturing such prisms, it is important that the dispersion of every prism beheld very closely to a specified amount so that'the dispersion of these prisms will agree with a standard dispersion scale used in different refractometers and will, therefore, be interchangeable. The great amount of highly skilled Work requiredto Vform accurately each of the nine angles between the refracting surfaces and carefully cement the three elements of each Amici prism together with a perfectly even layer of cement results in high costs for the instrument. Furthermore, vvariations in the index of refraction of the optical material from whichsuch prisms are made, as well as slight misalignments of the contiguous elements of the prism increases the difficulty of close control of dispersion. The Apresent practice of reforming the exit surface of the Amici prism at a revised angle so as to correct for all the accumulated errors is not eiective for high precision instruments since the dispersion in some parts of the spectrum is thereby rendered imperfect. To obtain an accurate specified maximum value of said dispersion, it is here proposed to provide a second Amici prism for adjusting the maximum ned amount of dispersion may be exactly obtained without recourse to expensive manufacturing methods.

It is, therefore, an object of this invention to provide yan improved refractometer having means for adjusting the maximum dispersion of the Frauenhaufer C-F lines of the visible spectrum accurately to a predetermined value.

It is another object to provide such a device in which its operating parts are protected from the injurious effects of the substances which are being tested or from mechanical damage.

It is also an object of this invention to provide an improved refractometer which is rugged and dependable and easy to operate under production conditions.

Further objects an advantages will be apparent in the details of construction and combination of parts by reference to the following description and accompanying drawings in which:

Fig. 1 is a sectional view taken substantially on the line I-I of Fig. 2 showing a refractometer embodying a preferred form of our invention,

Fig. 2 is a rear view of the refractometer, with parts broken away,

Fig. 3 is a sectional view of the instrument taken on the line 3--3 of Fig. 1, and A Fig. 4 is a perspective view of the refractometer on a reduced scale.

As shown in the drawings, the refractometer comprises an upright housing IU extending upwardly from a broad base portion I I and having `aninterior chamber I2 which is open at the bottom and extends substantially to the top of the housing. Large openings I3 and I4 are provided on opposite sides of the housing I0 to permit easy access to the mechanism in chamber I2. Cover plates I5 and I 6 are fixed respectively by any suitable means such as screws over the openings I3 and I4. Likewise. another opening I'I is provided in the rear part of housing Ill and a cover plate I8 is suitably fixed thereover which together with plates I 5 and IB totally enclose important working parts of the instrument as will be apparent hereinafter.

From the rear side of the housing IIJ projects a flange I9 on which a movable arm 2 I is pivoted by means of a spindle 22 which is iixed to said arm and is journaled in said flange. On the inner side of arm 2l there is lixed a substantially cylindrical prism box comprising a, semi-cylindrical part 23 held in any preferred manner such as the screws 24 to the arm. A complementary semi-cylindrical part 25 is hinged at 26 to the dispersion of the optical system so that a specistationary part 23. In order to hold the two parts pin 25 to permit placing the fluid specimen dill rectly on the contiguous interfaces-betweenflsaid specimen holding prisms 223 and 25S.

The free end of arm 2| .'extndsiintozchamber l2 through an elongated clearance sloti31ifo'rmed`- in the concavely shaped portion of rear wall 32 lofthe housing l5, and said free end is broadenedfanwise in a direction parallelfto. the cover'plates l l5 and I5 to form a scale sector 30. In the sector 3S of the arm and along its outer arcuate edge-,- is an arcuate opening 33L located at a constant radiusl abouttheaxis ofi'spirrdle`v 22: overlying opening 33on.onesideor'thesector 3ft-is a'.trans' parent glass scale plate 34 which is vretained against' the armbya pairfof holding/.clips 35 and tainer plate's'll a'vhich ltogether 'provide'fmeans for attaching'.tlie*scarleplateY tothe-sector' Oni plate 34, an areuatefsecton'scalef31ris suitably formed inl conformity with'. thee-shape* of; the;v opening 33' soxthe'scale mayxbezseen extending along the openings's' More'th'ani one scale may be prof` vided as shownin-Figfl; ifzdesired; and onescale ist-graduatediinfvalues of refractive index;

To," provide; eas-y. operationf off the instrument forlong periods o-lf use,` the actuating mechanism for swinging the arm 2l is operatedbya knob zwhicheis located :near: the-base off the -housing lilfso`A that the knobmayn be operated byi'afhand restingfon the saine surface as the instrument. Operatingy knob 38 is mounted-onfa-horizontal shaftA lwhich'isrsuitably journaled adjacent-'its ends inthe-oppositefside3 wallsV4 of the housing lil by means'ofbea-ring plates,` oneofwhich is'shown atfil infEig; 1, suitably secured tothe housing! An actuating screw 42, extending; Y.upwardly throughchamber I2: fronrshait 39, .crosses-*the arm 2| and isf'rotatably journaledf a-t itsends in bearings d'3, and-1M;-L Xedlto-l-thehousing: A' set of bevel gears 45 serve as a rotational'cennection between rtlie 'shaft 35. andfthefactuating screwf- 42.

Anf operating connection is'y providedi between the: actuating screw- 42e. andiarfm- 2l forY swinging the. arm:bywrotationofr the shaft 39. Theconne'ctiony coimirisesar nutrmember- -'threadedon the screwfl and-saidl nut isfheld against rotation L,

by al nut extension 41: which -engagesopposite sidesf ofa-'stationaryfguide rod 48. Rod 48 is suitablyvixedfat its vends'to thel housing Il)Y as by the screws illustrated' in:Fig. l. In the nut member'fi: is a slot 4l9- extending; substantially normal: tofthe'axis: ofvv the screw- 42, and a connecting-pin 5-l=is xed in=arm2l inafposition to protrudeunto' the slot-,49 andet it closely-so as to. provide at sliding connect-ion therewith inv a direction normal to the screw.v Y

Optical-:means areprovided .for reading the scaleilv comprisinga vlens 52 focusedY on the -scale and aninclined mirror 4surface' 53 for, directing thelineof sight towards'the scale. Lens 52- is secured inaneyepiece 5d which is slidably held for4 focusing` purposes; in astationary inclined tube Sii-suitablyr anchored in a curved front wall 55fon`. thehousing i0. Eyepiece 55 is positioned at the best elevation' and'v inclinationl for the comfort ofl the operator. The mirror. surface 53 is adjustably held in a bracket 51 extending from wall 56, and the bracket also mounts a transparent reticle disc 58 adjacent to the scale 31. A suitable iiducial mark, not shown, is formed in any desired manner upon the surface of the reticle disc 58.

To provide aconstant-illumination of the scale 31, a lamp -59 is'mounted on the housing in such a position that light from said lamp passes through the transparent scale plate 34 and enters the eyepieceli. Lamp 59 is held by a suitable bracket 6'! attached to housing I0 and is energized througlfiiaf'lead S2-*connected to a hold-down switchz which' in turn is connected to a transformer; 6&3? illustrated in Fig. 1 and a suitable source of"electricalf power.

1 The.-light.rays. emerging from the specimen holding prisms 28 and 29 form a dividing line in the ield of view of the instrument when the prism box is in the position of total reflection of saidarays andsaid :lineis achromatized- ,partly lby a compensating Amici fprism- 65 aligned on l the ,optical axisor the-instrument. PrismS-isflxed infany-preferredmanner within aprisrn tube'66 which is rotatably mountedfin-an inclinedbore 6:1 formed in. the rearI wall .32z of the housing. Longitudinal :motion of the` tube 66. is prevented .by angsuitable. means. such .f asa .spring-pressed ball.l f-locatedwithin a. blind7 hole in*l thewall of borea51, as shown-inFig. 3,y and-engaging ina periphera-lv-groove 685m said tube. Tubefis rotatedby a flanged andknurled compensating ring 69 fixed on the outer diameter of the tube by means of a set-screw or other preferred .holding device. Thexring 69 is markedon"` its radial Vface with Loppositely arranged twin. compensator scales 1lreading.inarbitrary'units from 0 to160././ .Together` Witha stationaryr index pointer 12;the

scalesf1l indicate the. angular positionof said tubeto theright-orileft ofiafmedian position during achromatization` of theline of.. total reect ance. infthe field of view/of-the instrument.

Aligned with the prism tube 66, is a telescope body tube. 13.-slidably.. held atV itsainnerend in the housing.. within the inclinedbore Gland similarly held,..at..the other. endirnan outer. inclinedbore 1li.V The fouter. endv of. the telescope, body.'` 13=pro jects outwardly from theputerwall.56rin-close proximity tothe eyepiecetanda telescope eyepiece 15.'is; slidably held in-said-outer end. Eyepiece.. 5l! isz provided withfameticle or v cross hair, not shown, locatedin.thegfocalplane of the eyepiece.v It..will.be notedthateyepieces 54 and 15 are, located close4 to each other so, thatonly-a minimum` amount of motion. of the-operator is necessaryinmaking a setting offthe dividingline andreading thev scale.

An essential. feature of.. this inventionis the vmeans. forfadjusting the; maximum; dispersion of thefoptical system of. the instrument toa speci.- fled dispersive Value. To provide this feature, thereis provideda second Amicwpvrism 16, called thel dispersion.,adjusting prism, which. is located on-the-optical axis ofY the instrument in operative alignment With-Amici prism 6.5. Both of the prisms. 5.5, and. 1 5. contribute. to, the. achromatiza.- tion'of the aforesaid boundary line and'each is made withr excessive dispersionv for the Frauenr haufe'r C--F lines of the,visible,spectrum above the amount requiredv by: the instrument. The amountV of maximum. dispersion is notv criticalr andit may vary through Wide 1imits,the only re.-A quirement being. that itqexceeda givenminimum amount which is at least as low as the aforesaid specied. dispersive value. Therefore,y since, the

tolerancefor each variable factor inthe prism elements is relatively large, a specified dispersion may easily .be provided by ordinaryA commercial methods. The important fact is that prism 16 is made rotatable relative to prism 65 so that-by rotating prism 18 the dispersion of the instrument may be reduced from its largest possible maximum value to an exact specified maximum value for the Frauenhaufer C--F lines of the visible spectrum. Relative rotation of the adjusting prism 16 may be accomplished in any preferred manner, such as by fixing it within'a sleeve 11 which is carried by and normally rotates with the prism tube 66 but it is also rotatable therein when angularly adjusting prism 16. "Limited angular movement between the prisms 65 and 16 is provided by manipulating a lock screw 18 which is threaded in sleeve 11 and projects freely through`an elongated peripheral slot 19 in the prism tube 66.

Light is provided for illuminating the eld of view of the telescope either by natural .or artificia1 light. A'constant artificial light source is afforded by using a lamp 8|] held in a suitable socket 8 j in the free end of a lamp arm 82.' Arm 82 is journaled on a cylindrical bearing 83 on arm I9 to rotate about said bearing so as to provide the correct incidence of light on the specimen holding prism 29. Lamp 80 is connected by a lead 84, extending into chamber l2, to the transformer 64 which is energized through a lead 85 and a switch 86 from an outside source of electrical power. A cylindrical lamp chimney 81, preferably made from a transparent material such as the commercial product Lucite and having a frosted outer surface to diffuse the light from the lamp 80, is suitably secured inthe lamp arm 82 so as to protect the lamp. If desired, natural light is obtained by use of a mirror 88 which is adjustably held in an inclined position to direct light from the sky into the telescope. The mirror is attached to the outer surface of chimney 81 by a pair of friction attachment clips 89 and 98, secured to the mirror in any desired manner such as screws and engaging in grooves 9| and 92 formed in the outer surface of the lamp chimney 81. Thus, the mirror may be inclined into any necessary position when in use, and may be swung out of the way when not in use or it may be easily taken off the chimney 81.

Before the refractometer is ready for operation, its maximum dispersion mus-t be adjusted to the specified amount of dispersion for the instrument to correct for the manufacturing errors in the Amici prisms 65 and 16 or other refractive errors in the optical system. This is accomplished in the following manner. A liquid test specimen having al1 its constants known, such as for instance, the refractive index for the D line of the visible spectrum and the dispersion characteristics, is placed between the specimen holding prisms 28 and 29 by introducing it through the ller channel 25. The lock screw 18 is loosened and turned so as to rotate the prism 16 to that end of slot 19 where greatest dispersion is secured. White light, emanating from either the lamp 88 or cast by the mirror 88, is directed through the test specimen to the telescope by moving the arm 82 until light appears in the field of view of said telescope. Operating knob 38 isvrotated to set the sector scale 31 at the aforesaid known value of refractive index of the test specimen as observed through eyepiece 54. A boundary line of total reflection consequently appears in the field of view of the telescope, and it is tentatively achromatized by rotating the prism tube 6B with the compensator ring 69 to the known setting on dials 1|` for the test specimen. Subsequently, the crosshairs in the telescope are brought into registry with the boundary line. After loosening lock screw 18, lthe adjusting prism 16 is rotated in a direction to attenuate the dispersion of the instrument until best achromatization of the boundary line is secured and the screw 18 is then tightenedto lock the parts in adjusted position. Now the refractometer is properly adjusted to oliset any dispersive errors in the structure of Amici prisms and 16 So that an accurate setting of the specified maximum dispersion for the C-F lines of the visible spectrum is obtained. With this adjustment made, the generaI operation of the instrument is the same as that of the wellknown Abbe refractometer.

From the foregoing, it will be seen that there is here provided a rugged, precision refractometer having improved operating means and means for regulating the maximum dispersion of the Amici prisms used in the instrument.

VEffective means are also provided for protecting the internal working parts thereof from damage caused by the materials being tested or from careless handling, all these features being in accordance with the stated objects of this invention. Although but certain forms of this refractometer have been shown and described in detail, it will be understood that other forms are possible and changes may be made in the arrangement and detailed structure of the parts and substitutions may be made without departing from the spirit of the invention as defined in the claims here appended.

We claim:

1. A refractometer comprising lan upright housing having an inclined wall and providing an interior chamber extending from top to bottom of the housing, an arm pivoted at one end about a lateral axis on the housing, the free end of said arm being movable in said chamber and extending toward said inclined wall, a scale on said free end, lens means held in said wall in optical alignment with said scale for reading the scale, a specimen holding prism carried by the arm near said axis, a prism tube mounted to rotate on its own axis in the housing adjacent said prism, means for rotating said tube, Amici type prism means fixed in said tube in optical alignment with the prism, a stationary telescope projecting outwardly through said inclined wall and optically aligned with the prism and Amici prism means for viewing the refracting characteristics of a specimen held on the prism, an adjustment knob rotatably mounted in the lower4 part of said housing, a rotatable screw member operatively connected to said knob and extending upwardly through said chamber across said arm, a nut threaded on said screw member, Said nut being operatively connected to said arm whereby the arm may be moved by rotation of the knob to vary the position of the specimen holding prism.

2. In a refractometer having a housing, an arm pivoted at one end about a lateral axis on said housing, a specimen holding prism held on said arm adjacent to said axis, an accurate scale carried by the arm and having its center of curvature on said axis, lens means on the housing and optically aligned with the prism to observe the refraction of light passing through a specimen held on the prism, means for moving the arm about its axis, the combination of a first prism tube rotatable on. its own axis within saidhous- 1ing,1ai'second`prism tube carriedv concentrically yby the. first .prism tube, means for rotationally adjusting one: prism; tube with respect tothe other,

and a rst/ and second Amiciprism fixed respecn 4tively within the iirst and second prism tubes-fin opticalv alignment with each other and with the specimen holding prismand said lens means, the Amici prisms together having av dispersion for the Frauenhaufer C and F lines of the visible 4spectrum in excess of the requirements of the refractometer whereby one Amici prism may be adjusted rotationally relative to the other Amici prism to reduce the maximum dispersion for -saidC and F lines to a predetermined lesser value.

position, the combined dispersion of the two Amici prisms for the Frauenhaufer C and F lines being greater than the dispersion, required for the refractometer whereby the dispersion produced by the two-Amici prisms may be adjusted for the refractometer.

JOHN W. FORREST. RAYMOND F. E. STEGEMA-N.

REFERENCES CITED The following references are of record in the iile of this patent:

FOREIGN PATENTS Country Date Great Britain Apr. 20, 1922 OTHER REFERENCES Bausch 8aA Lomb Publication: Abbe and Dipping Refractometer, D-202, IV, 1932, 20 page publication, pages 10 and 11 cited. (Copy in 88- 14 R.)

Gibb: Test on Optical Methods of VChemical Analysis, page 332, 1942, published by McGraw- Hill Book Co., Inc., New York, New York. (Copy in Division 7.)

Number Certificate of Correction Patent No. 2,574,734 November 13, 1951 JOHN W. FORREST ET AL.

It is hereby certified that error appears in the prinbed specification of the above numbered patent requiring correction as follows:

Column 6, line 69, for accurate read air-cuate; column 7, line 5, before Second insert a;

THOMAS F. MURPHY,

Assistant Gommsaoncr of Patens. 

